The present invention relates to an improved structure for a concentric tube type heat exchanger. Particularly, the invention relates to a concentric tube type recuperator having transparent outer tube walls to more effectively permit the transmission therethrough of radiant heat energy. The heat exchanger includes special heat absorbing material that is particularly positioned to absorb the radiant heat tranversing the transparent walls of the heat exchanger and transmit the abosorbed heat to cool air or other fluid flowing therethrough.
Recuperative heat exchange apparatus employing a series of concentric inner and outer tubes to transfer heat from hot exhaust gases to cool air to be used for combustion are well known in the heat exchange art. Examples of conventional heat exchangers of this type are to be found in U.S. Pat. No. 3,586,098 and U.S. Pat. No. 2,670,945 where a plurality of tubular heat exchangers are adapted to transmit heat from hot gas passing over the tubes to cool air flowing therethrough.
Typically, hot exhaust gases are admitted to the heat exchanger and then directed over the heat exchanger tubes while cool air to be heated is directed through annular passageways between the concentric tubes. A portion of the heat carried by the hot exhaust gas is then transmitted through the wall of the outer tube to the cool air flowing through the annular passageway between concentric tubes. As the outer tube is heated by the hot gas passing over it, heat is also radiated from the inner surface thereof to the inner tube which upon becoming heated, also transfers this heat to the cool air flowing thereby.
Present limits to the use of such apparatus are determined largely by the high temperature of the gases to which the tubes are exposed and this by corrosiveness of the gas to which the tubes are exposed. Therefore any measures to overcome these limitations must include increasing the heat resistance or the corrosion resistance of the tubes.